Seismic monitoring of an underground reservoir is generally carried out by coupling with the ground seismic sources and receivers in various combinations where the sources and/or the receivers are arranged at the surface or in the vicinity thereof, or in one or more wells through the explored formation. Series of seismic emission-reception cycles are carried out by changing each time the location of the seismic source in relation to the axis of the well where the receivers are installed, according to a technique referred to as walk-away technique, and by recording the arrivals at receivers R1 to Rn as a function of the propagation time t.
The seismic sources used are in most cases electromechanical vibrators: electrohydraulic, piezoelectric vibrators, etc. Piezoelectric type vibrators are for example described in patent FR-2,791,180 or in U.S. Pat. No. 5,360,951.
Monitoring of the evolution of reservoirs generally requires seismic monitoring operations at spaced-out intervals. In practice, the surface seismic equipment has to be reinstalled for each new seismic recording-session and the conditions of emission of the previous seismic operations are preferably reproduced.
A known method of monitoring a hydrocarbon reservoir or an underground fluid reservoir comprises using a monitoring system including receiving antennas formed by interconnecting seismic receivers, permanently installed respectively in shallow holes, with connection means to which cables linked to a seismic laboratory can be connected, and a vibrator truck that is moved in the field.
Using a mobile source such as a vibrator involves drawbacks, especially within the context of periodic monitoring of an underground storage reservoir. A movable source does not allow a sufficient reproducibility in time of the seismic waves emitted. It is very difficult to position the source exactly at the same points it occupied during the previous emission-reception cycles and, in the event that this point is exactly the same, to obtain exactly the same ground coupling coefficient. This represents a constraint for the (generally overloaded) site operator who has to facilitate its evolution, all the more so if the source used has to be triggered substantially in the same places as during the previous emission-reception cycles.
Patent FR-2,728,973 (U.S. Pat. No. 5,724,311) describes a method and a device intended for permanent seismic monitoring of an underground formation. In the context of regular long-term monitoring operations in an underground zone, a seismic emission-reception device is permanently installed on the development site so as to find each time stable operating conditions again: identical emission-reception locations, identical quality of coupling with the formation, etc. The device comprises a plurality of seismic sources (electromechanical vibrators for example) at fixed locations at the surface or buried at a shallow level, which are supplied and triggered by a central control and recording station. The seismic sources and the connection network can be buried or permanently installed at the surface, and associated with at least one array of receivers permanently coupled with the ground at the surface or with the wall of at least one well drilled through the underground zone. All these permanently installed sources whose coupling with the surrounding formations remains stable and this supply network, at least partly buried, whose surface coverage area is limited, allow to carry out a series of seismic monitoring operations under stable operating conditions, without any risk of incompatibility with the development site activities.
Patent FR-2,728,973 (U.S. Pat. No. 5,724,311) describes another seismic monitoring device intended for permanent monitoring of an underground formation by means of one or more emission-reception sets comprising each a source such as a vibrator and a receiving antenna consisting of a plurality of elastic wave receivers such as geophones and/or hydrophones lowered in a well and coupled with the formation. The seismic source is arranged in a cavity close to the location of the antenna or formed by widening the section of this well in its upper part. The receivers and the source are connected to an outside control and signal acquisition station. The operations allowing setting of these devices are relatively simple and the ground coverage area in the various wells is reduced, which facilitates their integration in reservoir development sites.
By means of these permanently installed sources easy to integrate in reservoir development sites or fluid storage sites, and whose coupling quality with the surrounding formations is known and stable, a series of seismic monitoring operations can be carried out under similar operating conditions. The seismic trace sets can be usefully compared and their differences reflect the changes occurred in the formations, and not disturbances linked with climatic variations for example.
However, monitoring operations require permanent coupling of an often large number of seismic sources, which makes the monitoring installation relatively expensive.